Offshore structures in the form of large, permanent offshore platforms are erected on the ocean floor to drill wells therefrom to develop oil and gas formations. After the permanent platform has been installed, the necessary drilling equipment including a large drilling derrick is positioned on the platform for drilling a number of wells one at a time. After one well is drilled, the drilling derrick is skidded to a new position and a second or third well is drilled. After the selected number of wells have been drilled from the platform, the drilling equipment including the derrick is removed and placed on barges and towed to another location or to shore. Production equipment is then installed on the platform and the wells are produced after connecting the platform to shore by means of a pipeline.
From time to time it may become desirable to drill an additional well or two from the same permanent platform. However, the drilling equipment and derrick must be reinstalled on the platform to drill these additional wells. A derrick is also necessary to carry out the completions and workovers of deep high pressure wells offshore. However, it is very time consuming to hoist the various equipment, including the derrick, aboard the permanent platform and to assemble the equipment to carry out further drilling or workovers.
The necessary equipment has in the past been shifted off specially rigged jack-up rigs brought alongside the permanent platform. However, only a small fraction of the jack-up rigs allow the continuous load-bearing height adjustments necessary to align the deck levels of the respective platforms. The scarcity of these continuously adjusting jack-up rigs means that such equipment is not always available at the time needed in order to carry out these operations and may remain unavailable for extended periods of time. By contrast, the vast majority of the existing jack-up rig fleet locks off at load-bearing platform levels at increments of five or six feet and are unsuitable at the present time for skidding the drilling equipment to the permanent platform Further, the expense of refitting the jacking mechanisms for these jack-up rigs to provide for continuous height adjustments would require major reworking of substantial components of very expensive offshore vessels. Such major overhauling is economically unattractive in relation to the cost and inconvenience of hoisting unassembled drilling equipment to the permanent platform for assembly there. Thus, there is a substantial need for an apparatus and method to permit use of the existing fleet of jack-up rigs for installing assembled drilling equipment onto permanent platforms.